A directional coupler is used as a microwave circuit for separating a progressive wave from a reflected wave in various fields. In an ideal directional coupler, the progressive wave and the reflected wave are completely separated from each other, and hence only the progressive wave appears at a coupling port while only the reflected wave is generated at the isolation port. Therefore, a directionality that is a power ratio between the progressive wave and the reflected wave becomes infinite.
If a high directionality is desired to be realized, it is necessary to match phase speeds of even and odd modes. However, a microstrip line that is widely used as a micro circuit is a heterogeneous medium line, and hence a difference in wavelength shortening ratio occurs between the even mode and the odd mode. Therefore, the reflected wave may leak out to the coupling port, which causes a problem that the directionality expressed by a difference between a power generated at the coupling port and a leakage power at the isolation port is deteriorated.
Therefore, some methods have been proposed for improving the directionality, which includes a method of providing a feedback line (see, for example, Non-patent Document 1), a method of processing portions of a main line and a coupling line facing each other (see, for example, Patent Document 1), a method of providing a floating conductor to a coupling portion of the coupling line (see, for example, Non-patent Document 2), and the like.
FIG. 10 is a schematic diagram of a conventional directional coupler in Non-patent Document 1. The directional coupler in Non-patent Document 1 includes feedback lines 103 and 104, which are respectively disposed between input and output terminals of a main line 101, and between the isolation port 108 and the coupling port 107, and hence the directionality is improved.
More specifically, the improvement of the directionality is intended as follows. The main line 101 and a coupling line 102 facing each other are connected to the feedback lines 103 and 104, respectively. An RF signal received from an input terminal 105 is led to an output terminal 106 via the main line 101. Further, the main line 101 is coupled with the coupling line 102, and hence the signal received from the input terminal 105 is led to the coupling port 107.
Then, the phase speeds of the even mode and the odd mode are matched with each other by the feedback lines 103 and 104. Thus, the signal is not led to an isolation port 108, and hence the improvement of the directionality is intended.
In addition, FIG. 11 is a schematic diagram of a conventional directional coupler in Patent Document 1. This directional coupler of Patent Document 1 improves the directionality by disposing portions 115 and 116 at both ends of the coupling portion of a coupling line 110 so as to increase capacitance of a main line 119, and by letting facing portions 117 of the main line 119 and the coupling line 110 have inductances.
More specifically, the improvement of the directionality is intended as follows. A coupling portion 118 of the main line 119 and the coupling line 110 has portions 115 and 116 for increasing capacitance of the main line 119 at both ends of the coupling line 110, and a portion for having inductance at the facing portions 117 of the main line 119 and the coupling line 110. An RF signal received from an input terminal 111 is led to an output terminal 112 via the main line 119.
Further, the main line 119 is coupled with the coupling line 110, and hence the signal received from the input terminal 111 is led to a coupling port 113. The directional coupler of Patent Document 1 makes phase speeds of the even mode and the odd mode match with each other by means of the portions 115 and 116 that increase the capacitance of the main line 119 and are disposed on both ends of the coupling portion 118 of the coupling line 110, and the facing portions 117 of the main line 119 and the coupling line 110 having inductance. Thus, the signal is prevented from being led to an isolation port 114, and hence improvement of the directionality is intended.
In addition, FIG. 12 is a schematic diagram of a conventional directional coupler in Non-patent Document 2. This directional coupler in Non-patent Document 2 compensates for a phase difference between the even mode and the odd mode by providing floating conductors 127 to a coupling portion of a coupling line 126, and hence the directionality is improved.
More specifically, the improvement of the directionality is intended as follows. The floating conductors 127 are disposed in a coupling portion of a main line 125 and the coupling line 126. An RF signal received from an input terminal 121 is led to an output terminal 122 through the main line 125. Further, the main line 125 is coupled with the coupling line 126, and hence the signal received from the input terminal 121 is led to a coupling port 123.
Then, periodical slits provided to the coupling portion increase a distributed inductance of the odd mode mainly. In addition, the floating conductor inserted in the coupling portion affects almost only distributed capacitance of the odd mode. Therefore, the phase speeds of the even mode and the odd mode can be matched with each other by adjusting sizes of the slit and the floating conductors 127. Thus, it is possible to prevent the signal from being led to an isolation port 124, and hence the improvement of the directionality is intended.